Polysaccharides produced by plants are useful for a variety of home and industrial applications. Plant gums and starches, for example, are used extensively in food and pharmaceutical industries, due to their emulsifying, stabilizing, thickening, and gel-forming properties (see, for example, Sandford and Baird, in The Polysaccharides (Aspinall, ed.), 2:411 (Academic Press 1983); Azczesniak, in Gums and Stabilisers for the Food Industry (Phillips et al., eds), 3:311 (Elsevier Applied Science Publishers 1986)). It would therefore be beneficial to alter plant polysaccharide synthesis to improve the quantities of polysaccharides or to provide new types of polysaccharides. Modifications of polysaccharide synthesis also can lead to improved plants. For example, increasing the amount of cell wall polysaccharide during development would improve resistance of developing seeds to pathogens and insects resulting in improved crop yields.
UDP-glucose is a nucleotide sugar that occupies a central position in plant metabolic pathways, including the synthesis of polysaccharides. UDP-glucose serves both as a precursor in sucrose synthesis and in the formation of those sugar nucleotides required for the synthesis of cell wall components. For example, UDP-glucose metabolism leads to amino sugars, cellulose, sucrose, fructans, and other non-cellulosic polysaccharides. In addition, UDP-glucose can substitute for ADP-glucose as a starch synthase substrate, at least in the waxy maize phenotype.
UDP-glucose dehydrogenase (UDPGdH) [EC 1.1.1.22] is an NAD+-linked, four-electron transferring oxidoreductase that converts UDP-D-glucose (UDP-G) to UDP-D-glucuronic acid (UDP-GA) by two oxidation reactions in which UDP-6-aldehydo-D-glucose is an intermediate (Hempel et al., Protein Science 3:1074, 1994). UDP-GA is a precursor for sugar nucleotides, which are required for the biosynthesis of various components of hemicellulose, including arabinans, arabinogalactans, glucuronoarabinoxylans, rhamnogalacturonans, xylans, and xyloglucans. Evidence indicates that UDPGdH catalyzes the rate-limiting step in the synthesis of these cell wall precursors (Witt, Journal of Plant Physiology 140:276, 1992). UDPGdH is also centrally involved in the production of a variety of exopolysaccharide gums including xanthan gum and a variety of non-commercial gums produced by Streptococci (Ashtaputre and Shah, Current Microbiology 31:234, 1995); Lin et al., Biochemical & Biophysical Research Communications 207:223, 1995).
Since it is difficult to control the quantity and composition of hemicellulosic substances, there are problems with extraction of other useful plant-derived fibers and substances. Further, many naturally-occurring hemicellulosic substances and other polysaccharides derived from UDPGdH products are not useful commercial products due to their insufficient quantity and varying composition. Accordingly, there is a need for a means to improve and regulate the production of plant polysaccharides.
The present invention exploits novel genes that are highly and specifically expressed in developing plants to alter the regulation of UDPGdH activity, thus providing control of plant polysaccharide synthesis.